OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 1 followed by the digits 01 is prime (see Example section).
a(23) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 81w01.
EXAMPLE
3 is in this sequence because (73*10^3 - 91)/9 = 8101 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 3, 8101;
a(3) = 4, 81101;
a(4) = 6, 8111101;
a(5) = 7, 81111101; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(73*10^# - 91)/9] &]
ParallelMap[If[PrimeQ[(73*10^# - 91)/9], #, Nothing] &, Range@100000] (* Robert G. Wilson v, Oct 31 2017 *)
PROG
(PARI) isok(n) = isprime((73*10^n - 91)/9); \\ Michel Marcus, Nov 01 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 31 2017
STATUS
approved